Traffic management is critical to the successful operation of cell-based transmission in ATM-based networks. Cell-based transmission systems are subject to congestion caused by unpredictable statistical fluctuations of traffic flows and fault conditions within the network. Congestion of such systems refers to the state of network elements, such as switches, concentrators, cross-connects and transmission links, in which the network is not able to meet the negotiated network performance objectives for the already established connections and/or for the new connection requests. In the absence of effective traffic management, traffic loads from users can exceed the capacity of the network, resulting in an overall degradation of network performance and the loss of data.
In order to manage transfer capacity needs and satisfy network performance objectives, reliable and effective traffic control functions are required. Traffic control in such systems refers to the set of actions taken by the network to avoid congested conditions. The primary role of traffic control is to protect the network while providing users with the Quality of Service (QOS) objectives agreed upon between each user and the network for the particular ATM connection. Traffic control is also important to optimize the use of network resources.
In an ATM-based network, the traffic control strategy is based on determining whether an ATM connection (VPC or VCC) can be accommodated at the User Network Interface (UNI) and negotiating the performance parameters that will be supported. Traffic parameters describe the traffic characteristics of an ATM connection. For example, traffic parameters may describe peak cell rate (PCR), cell delay variation (CDV), burst tolerance (BT), sustainable cell rate (SCR), peak duration and/or source type (e.g. constant bit rate and variable bit rate video, bulk data, voice, e-mail). When a user requests a new ATM connection the user must specify the traffic parameters for that connection. The user specifies the traffic parameters by selecting a QOS from the QOS classes provided by the network. The connection is accepted by the network if the necessary resources are available to support the traffic level while maintaining the agreed upon QOS for existing connections.
Where a connection is established, the network and the user enter into a "traffic contract". The traffic contract refers to the negotiated characteristics of an ATM connection at the private or public UNI, and includes the conformance definition that is used to unambiguously specify the conforming cells of the connection. The agreed upon QOS is provided by the network for so long as the user complies with the traffic contract.
The conformance definition used to specify conforming cells of an ATM connection at the UNI is based on the generic cell rate algorithm (GCRA) described in ITU-T Recommendation I.371, prepared by ITU-T Study Group XVIII and published by the International Telecommunication Union (ITU) in March 1993 (referred to here as "ITU-T I.371"). The GCRA is used to determine whether traffic conforms to the traffic contract. Two equivalent algorithms are identified in ITU-T I.371, namely the virtual scheduling algorithm and a continuous-state leaky bucket algorithm. In making these algorithms realizable in hardware, finite word lengths must be used to represent arrival times and other traffic parameters. As a result, a specific traffic control mechanism is susceptible to committing errors by taking policing actions on a compliant (i.e. conforming) connection, in other words declaring a cell as non-compliant (non-conforming) although the connection is actually compliant (conforming). It can also fail to take the appropriate policing actions on a non-compliant connection. Inappropriate actions on a compliant or non-compliant connection are part of the overall network performance degradation.
A significant improvement in the prior art is found in our co-pending application Ser. No. 08/568,379 filed Dec. 6, 1995 and entitled TRAFFIC CONTROLLER FOR CELL-BASED TRANSMISSION, which is incorporated herein by reference. In this co-pending application there is disclosed a traffic control device and method for identifying cells as either conforming or non-conforming in a cell-based transmission system. The device disclosed includes an n-bit word length counter having an input coupled to a source of clock pulses, a parallel n-bit word length gate coupled to a parallel output of the n-bit word-length counter and having a gate input coupled to a source of cell arrival signals. A processor is also provided, coupled to an output of the gate and which determines on a periodic basis whether an arriving cell is compliant or non-compliant with traffic parameters. The processor includes means for determining if a theoretical arrical time, TAT, is less than an arrival time for a k.sup.th cell, t.sub.a (k), where k is an integer, or an (n-1).sup.th bit of t.sub.a (k), namely t.sub.a (k) [n-1], is zero and an (n-1).sup.th bit of an arrival time for a (k-1).sup.th cell, namely t.sub.a (k-1)[n-1], is one; means for setting TAT equal to t.sub.a (k) if either of the tests are true; and means for determining if TAT is greater than t.sub.a (k)+L, where L is a limit value for cell time of arrival variation, if all of the tests are not true. The processor further includes means for identifying a cell non-conforming if TAT is greater than t.sub.a (k)+L, and means for incrementing TAT by a numeric value I and identifying a cell conforming if TAT is less than or equal to t.sub.a (k)+L or if either of the earlier tests between TAT and t.sub.a (k) are true. The traffic control device disclosed provides a useful mechanism for identifying conforming and non-conforming cells on a periodic basis where there is low cell delay variation.
Associated with the problem of determining compliance is the need, particularly in high-speed AT-based networks where line rates may exceed 800 Mbps, for a traffic control system adaptive to high cell delay variation tolerances and which is capable of continuous monitoring and tracking boundary conditions in a finite word length architecture so as to provide greater precision in identifying conforming and non-conforming cells.
Accordingly, it is an object of the invention to provide apparatus and method for traffic control in a cell-based network which fulfills the above needs in the art and which avoids false identifications of non-conforming and conforming cells.